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Sept. 2, 2008 -- A new study suggests that children born from older fathers are at increased risk of developing bipolar disorder.

Earlier research has shown a link between older paternal age and risk for autism and schizophrenia. The new findings appear in the September issue of Archives of Psychiatry.

Overall, children born to fathers in their mid-50s and older were found to have a 37% higher risk for bipolar disorder than children born to dads in their early 20s.

The risk of developing the mood disorder before the age of 20 was roughly 2.5-times greater for children born to men age 50 and older than for children born to men between the ages of 20 and 24.

While characterizing this increase in risk as "quite strong," researcher Emma M. Frans, MmedSc, of Stockholm's Karolinska Institute tells WebMD that the relative risk at the individual level is still very small.

"There are very few men having children at this age, and most of the children born to these men will be healthy," she says.

Not Much Known About Bipolar Causes

According to the National Institute of Mental Health, about 5.7 million American adults have bipolar disorder, a serious mental illness characterized by dramatic, episodic mood swings.

While the mood disorder tends to run in families, suggesting a genetic link, little else is known about the causes of bipolar disorder.

Because older paternal age has been found to be a risk factor for other genetically influenced mental illnesses, such as schizophrenia, Frans and colleagues explored its role in bipolar disorder.

Using data from a nationwide Swedish health registry, they identified close to 13,500 people with a diagnosis of bipolar disorder. Each was randomly matched to five people without the disorder who were the same sex and born in the same year for comparison.

After taking into account maternal age and several other potential influences on risk, the researchers concluded that the offspring of men 55 years of age and older were 1.37 times more likely to have a diagnosis of bipolar disorder than the offspring of men between the ages of 20 and 24.

Older maternal age was associated with a slight, but nonsignificant, overall increase in risk, but no association was seen between maternal age and the risk for a bipolar diagnosis before age 20.

Greater Age Means More Mutations

The fact that paternal age appears to be a more important risk factor for bipolar disorder than maternal age suggests that genetic mutations in sperm may be to blame, Frans says.

Men add more mutations to the gene pool than women because their reproductive cells continue to divide throughout their lives. Women have only about 23 divisions in the cells that produce their eggs, and these divisions occur before birth, the researchers note.

More divisions mean more potential mutations or DNA damage that could be driving the increased risk for bipolar disorder and other genetically influenced mental disorders.

According to one analysis cited by the researchers, by the time a man reaches the age of 20 the cells that create sperm will have passed through 200 divisions. By age 40, about 660 divisions have occurred.

Male fertility expert Harry Fisch, MD, tells WebMD that researchers are only just beginning to understand the impact of paternal age on child health.

Fisch directs the Male Reproductive Center at New York-Presbyterian Hospital/Columbia University Medical Center. He is also the author of the book The Male Biological Clock.

"What we know probably represents just the tip of the iceberg," he says. "Until just a few years ago, there was not much research in this area. But it is important that we understand this because so many more men are having children later in life."